High dynamic range (“HDR”) image sensors are useful for many applications. In general, ordinary image sensors, including for example charge coupled device (“CCD”) and complementary metal oxide semiconductor (“CMOS”) image sensors, have a dynamic range of approximately 70 dB dynamic range. In comparison, the human eye has a dynamic range of up to approximately 100 dB. There are a variety of situation in which an image sensor having an increased dynamic range is beneficial. For example, image sensors having a dynamic range of more than 100 dB are needed in the automotive industry in order to handle different driving conditions, such as driving from a dark tunnel into bright sunlight. Indeed, many applications may require image sensors with at least 90 dB of dynamic range or more to accommodate a wide range of lighting situations, varying from low light conditions to bright light conditions.
One known approach for implementing HDR image sensors is to use a combination of a photodiodes in each pixel. One of the photodiodes can be used to sense bright light conditions while another photodiode can be used to sense low light conditions. In this approach, the photodiode used to sense bright light is typically smaller (having a smaller light exposure area) than the photodiode used to sense low light conditions. However, this approach requires an asymmetric layout that tends to increase costs. In addition to increasing cost, asymmetric fabrication of the photodiodes in each pixel includes optical asymmetry that may introduce image light ray angle separation. Image light ray angle separation can cause asymmetric blooming, crosstalk, and other undesirable effects, especially when the image light is angled relative to the face of the image sensor.